Kemp Elimination Reaction Catalyzed by Electric Fields

The Kemp elimination reaction is the most widely used in the de novo design of new enzymes. The effect of two different kinds of electric fields in the reactions of acetate as a base with benzisoxazole and 5-nitrobenzisoxazole as substrates have been theoretically studied. The effect of the solvent reaction field has been calculated using the SMD continuum model for several solvents; we have shown that solvents inhibit both reactions, the decrease of the reaction rate being larger as far as the dielectric constant is increased. The diminution of the reaction rate is especially remarkable between aprotic organic solvents and protic solvents as water, the electrostatic term of the hydrogen bonds being the main factor for the large inhibitory effect of water. The presence of an external electric field oriented in the direction of the charge transfer (z axis) increases it and, so, the reaction rate. In the reaction of the nitro compound, if the electric field is oriented in an orthogonal direction (x axis) the charge transfer to the NO₂ group is favored and there is a subsequent increase of the reaction rate. However, this increase is smaller than the one produced by the field in the z axis. It is worthwhile mentioning that one of the main effects of external electric fields of intermediate intensity is the reorientation of the reactants. Finally, the implications of our results in the de novo design of enzymes are discussed.     

TITAN: A Code for Modeling and Generating Electric Fields—Features and Applications to Enzymatic Reactivity

We present here a versatile computational code named “elecTric fIeld generaTion And maNipulation (TITAN),” capable of generating various types of external electric fields, as well as quantifying the local (or intrinsic) electric fields present in proteins and other biological systems according to Coulomb's Law. The generated electric fields can be coupled with quantum mechanics (QM), molecular mechanics (MM), QM/MM, and molecular dynamics calculations in most available software packages. The capabilities of the TITAN code are illustrated throughout the text with the help of examples. We end by presenting an application, in which the effects of the local electric field on the hydrogen transfer reaction in cytochrome P450 OleT_JE enzyme and the modifications induced by the application of an oriented external electric field are examined. We find that the protein matrix in P450 OleT_JE acts as a moderate catalyst and that orienting an external electric field along the Fe─O bond of compound I has the biggest impact on the reaction barrier. The induced catalysis/inhibition correlates with the calculated spin density on the O-atom. © 2019 Wiley Periodicals, Inc.     

Switch chemistry at cryogenic conditions: quantum tunnelling under electric fields

While the influence of intramolecular electric fields is a known feature in enzymes, the use of oriented external electric fields (EEF) to enhance or inhibit molecular reactivity is a promising topic still in its infancy. Herein we will explore computationally the effects that EEF can provoke in simple molecules close to the absolute zero, where quantum tunnelling (QT) is the sole mechanistic option. We studied three exemplary systems, each one with different reactivity features and known QT kinetics: π bond-shifting in pentalene, Cope rearrangement in semibullvalene, and cycloreversion of diazabicyclohexadiene. The kinetics of these cases depend both on the field strength and its direction, usually giving subtle but remarkable changes. However, for the cycloreversion, which suffers large changes on the dipole through the reaction, we also observed striking results. Between the effects caused by the EEF on the QT we observed an inversion of the Arrhenius equation, deactivation of the molecular fluxionality, and stabilization or instantaneous decomposition of the system. All these effects may well be achieved, literally, at the flick of a switch.

Adding an external electric field to reactions driven by quantum mechanical tunneling brings a whole new dimension to the idea of switch chemistry.     

纳米通道内酶组装及其催化反应研究进展

研究酶的组装和催化反应不仅有利于探索生命活动的本质,同时对开发酶在工业合成、分析检测、疾病治疗等领域的实际应用价值具有重要的指导意义. 研究发现,酶的有效固定和有序组装是保持酶活性、酶促反应的稳定性和对酶催化过程进行控制的重要途径,而在纳米通道内进行单酶或多酶的有序组装,利用纳米通道的限域效应可有效保持酶的构型进而提高酶催化反应的选择性和催化效率,增强酶级联反应的动力学进程. 本文概述了近年来基于纳米通道的酶反应器在生物传感领域的研究进展,着重描述纳米通道限域空腔内酶的组装方法、酶催化反应及其动力学机制,并展望了基于纳米通道的酶反应器的应用前景.     

Oriented Electric Fields Accelerate Diels–Alder Reactions and Control the endo/exo Selectivity

Herein we demonstrate that an external electric field (EEF) acts as an accessory catalyst/inhibitor for Diels–Alder (DA) reactions. When the EEF is oriented along the “reaction axis” (the coordinate of approach of the reactants in the reaction path), the barrier of the DA reactions is lowered by a significant amount, equivalent to rate enhancements by 4–6 orders of magnitude. Simply flipping the EEF direction has the opposite effect, and the EEF acts as an inhibitor. Additionally, an EEF oriented perpendicular to the “reaction axis” in the direction of the individual molecule dipoles can change the endo/exo selectivity, favouring one or the other depending on the positive/negative directions of the EEF vis‐à‐vis the individual molecular dipole. At some critical value of the EEF along the “reaction axis”, there is a crossover to a stepwise mechanism that involves a zwitterionic intermediate. The valence bond diagram model is used to comprehend these trends and to derive a selection rule for EEF effects on chemical reactions: an EEF aligned in the direction of the electron flow between the reactants will lower the reaction barrier. It is shown that the exo/endo control by the EEF is not associated with changes in secondary orbital interactions.     

Modeling of Solute-Solvent Interactions Using an External Electric Field—From Tautomeric Equilibrium in Nonpolar Solvents to the Dissociation of Alkali Metal Halides

An implicit account of the solvent effect can be carried out using traditional static quantum chemistry calculations by applying an external electric field to the studied molecular system. This approach allows one to distinguish between the effects of the macroscopic reaction field of the solvent and specific solute–solvent interactions. In this study, we report on the dependence of the simulation results on the use of the polarizable continuum approximation and on the importance of the solvent effect in nonpolar solvents. The latter was demonstrated using experimental data on tautomeric equilibria between the pyridone and hydroxypyridine forms of 2,6-di-tert-butyl-4-hydroxy-pyridine in cyclohexane and chloroform.     

How Do Metal Ions Modulate the Rate-Determining Electron-Transfer Step in Cytochrome P450 Reactions?

Cytochrome P450 (CYP450) enzymes play important roles in maintaining human health and their reaction rates are dependent on the first electron transfer from the reduction partner. Interestingly, experimental work has shown that this step is highly influenced by the addition of metal ions. To understand the effect of external perturbations on the CYP450 first reduction step, we have performed a computational study with model complexes in the presence of metal and organic ions, solvent molecules, and an electric field. The results show that these medium-range interactions affect the driving force as well as electron-transfer rates dramatically. Based on the location, distance, and direction of the ions/electric field, the catalytic reaction rates are enhanced or impaired. Calculations on a large crystal structure with bonded alkali metal ions indicated inhibition patterns of the ions. Therefore, we predict that the active forms of the natural CYP450 isozymes will not have more than one alkali metal ion bound in the second-coordination sphere. As such, this study provides an insight into the activity of CYP450 enzymes and the effects of ions and electric field perturbations on their activity.     

Estimations of electric field effects on the oxygen reduction reaction based on the density functional theory

By varying the external electric field in density functional theory (DFT) calculations we have estimated the impact of the local electric field in the electric double layer on the oxygen reduction reaction (ORR). Potentially, including the local electric field could change adsorption energies and barriers substantially, thereby affecting the reaction mechanism predicted for ORR on different metals. To estimate the effect of local electric fields on ORR we combine the DFT results at various external electric field strengths with a previously developed model of electrochemical reactions which fully accounts for the effect of the electrode potential. We find that the local electric field only slightly affects the output of the model. Hence, the general picture obtained without inclusion of the electric field still persists. However, for accurate predictions at oxygen reduction potentials close to the volcano top local electric field effects may be of importance.     

Effect of weak pulse electric fields on surface properties and destructive activity of <Emphasis Type="Italic">Pseudomonas</Emphasis> bacteria

The effect of weak pulse electric fields on the destructive activity and surface properties of Pseudomonas bacteria is studied. It is revealed that the physiological response of bacteria on their treatment by external pulse fields depends on field parameters and is accompanied by the changes in the values of electrokinetic potential and hydrophobicity. The treatment of bacteria by the electric field with pulse duration of 1–10 ms and frequency of 100–500 Hz tends to increase by 20–30% the destruction of NaAg(CN)₂. It is suggested that changes in surface properties and the enhancement of biochemical activity of bacteria are based on the metabolic reaction of the cell related to the stimulation or suppression of the bacteria respiration upon the imposition of external field.     

Theoretical study of the adsorption and dissociation of oxygen on Pt(111) in the presence of homogeneous electric fields

The effect of homogeneous electric fields on the adsorption energies of atomic and molecular oxygen and the dissociation activation energy of molecular oxygen on Pt(111) were studied by density functional theory (DFT). Positive electric fields, corresponding to positively charged surfaces, reduce the adsorption energies of the oxygen species on Pt(111), whereas negative fields increase the adsorption energies. The magnitude of the energy change for a given field is primarily determined by the static surface dipole moment induced by adsorption. On 10-atom Pt(111) clusters, the adsorption energy of atomic oxygen decreased by ca. 0.25 eV in the presence of a 0.51 V/A (0.01 au) electric field. This energy change, however, is heavily dependent on the number of atoms in the Pt(111) cluster, as the static dipole moment decreases with cluster size. Similar calculations with periodic slab models revealed a change in energy smaller by roughly an order of magnitude relative to the 10-atom cluster results. Calculations with adsorbed molecular oxygen and its transition state for dissociation showed similar behavior. Additionally, substrate relaxation in periodic slab models lowers the static dipole moment and, therefore, the effect of electric field on binding energy. The results presented in this paper indicate that the electrostatic effect of electric fields at fuel cell cathodes may be sufficiently large to influence the oxygen reduction reaction kinetics by increasing the activation energy for dissociation.     

Calculation of direct and indirect excitons in GaAs–Ga1−xAlxAs coupled double quantum wells: Electric and magnetic fields and hydrostatic pressure effects

A study of the electronic and optical properties of coupled double quantum wells is presented. Within the framework of the effective mass and parabolic-band approximations we have calculated the electron–hole and photoluminescence energy transitions under simultaneous effects of electric and magnetic fields. For that purpose, a variational procedure has been used, taking into account the effect of hydrostatic pressure. The electric field is taken to be oriented along the growth direction of the heterostructure whereas for the magnetic field both in-plane and in-growth directions have been considered. The results show that hydrostatic pressure is a useful tool to tune the direct and indirect exciton transitions in such heterostructures. It is shown that the photoluminescence peak energy transitions strongly depend on the external fields and hydrostatic pressure studied here. Furthermore, our numerical outcome is in good agreement with previous experimental findings at zero pressure in double quantum wells under applied electric and magnetic fields.     

Global and local relative convexity and oriented relative convexity; application to molecular shapes in external fields

The concepts of global and local relative convexity and oriented relative convexity are described and proposed as tools for the characterization of molecular shapes. The usual concept of convexity is a special case of the generalization described. Oriented relative convexity is suitable for the characterization of molecular shapes in external fields, such as magnetic fields or fields representing cavity regions of various enzymes or zeolite catalysts. Potential applications include new approaches to computer-based drug design and molecular engineering.     

Response of Bi-disperse Polyelectrolyte Brushes to External Electric Fields——A Numerical Self-consistent Field Theory Study

The self-consistent field theory has been employed to numerically study the response of bi-disperse flexible polyelectrolyte (PE) brushes grafted on an electrode to electric fields generated by opposite surface charges on the PE-grafted electrode and a second parallel electrode. The numerical study reveals that, under a positive external electric field, the shorter and negatively charged PE chains are more responsive than the longer PE chains in terms of the relative changes in their respective brush heights. Whereas under a negative external electric field, the opposite was observed. The total electric force on the grafted PE chains was calculated and it was found that, under a positive external electric field, the magnitude of the total electric force acting on one shorter PE chain is larger than that on one longer PE chain, or vice versa. The underlying mechanism was unraveled through analyzing the total electric field across the two oppositely charged electrodes.     

外部电场下水分子间静电相互作用的分子动力学研究

本文利用分子动力学模拟探讨了不同外电场下,液态水的分子间作用及分子排布的变化. 在不同外电场下,O…O原子间的径向分布函数差别很小,但是单个水分子的偶极矩的取向变化却很大. 当外电场为0时,单个水分子偶极取向的范围很宽（30-150度）. 与此同时,本文给出了局域诱导电场随着位置的变化关系图. 当外加电场增强时,局域的诱导电场强度也随之增加. 由于电场下偶极矩有序性的增加,局域诱导的静电相互作用能显著增加. 计算结果表明,相对介电常数随着电场强度的增加而呈现指数衰减的变化形式. 这一变化趋势可以用来理解不同电化学环境下,静电相互作用和局域诱导电场的变化.     

Temperature-dependence of time-dependent friction and electric field fluctuations

Molecular-dynamics simulations of a model dipolar system containing two ions have been carried out to study the effect of temperature on the time-dependent friction (ζ(t)) on the solute ions as well as the electric fields in the vicinity of the ions. It is found that with an increase in temperature, (i) the magnitude of ζ (0) increases and (ii) the electric field time correlation functions decay more rapidly at higher temperatures due to faster reorientations and translational diffusion of the molecules. The effects of the size of the system on these properties have also been investigated.     

A greener catalyst for hydroboration of imines—external electric field modify the reaction mechanism

Usually, an extra catalyst (for example, the transition metal complexes) need to be used in catalyzing hydroboration, which involved the cost, environment, and so forth. Here, a greener and controllable catalyst—external electric field (EEF) was used to study its effect on hydroboration of N-(4-methylbenzyl)aniline (PhN═CHPhMe) with pinacolboane (HBPin). The results demonstrated that EEF could affect the barrier heights of both two pathways of this reaction. More significantly, flipping the direction of EEF could modify the reaction mechanism to induce a dominant inverse hydroboration at some field strength. That is to say, oriented EEF is a controlling switch for the anti- or Markovnikov hydroboration reaction of imines. This investigation is meaningful for the exploration of greener catalyst for chemistry reaction and guide a new method for the Markovnikov hydroboration addition. © 2019 Wiley Periodicals, Inc.     

Multi-Domain versus Single-Domain: A Magnetic Field is Not a Must for Promoting Spin-Polarized Water Oxidation

The reaction kinetics of spin-polarized oxygen evolution reaction (OER) can be enhanced by ferromagnetic (FM) catalysts under an external magnetic field. However, applying a magnetic field necessitates additional energy consumption and creates design difficulties for OER. Herein, we demonstrate that a single-domain FM catalyst without external magnetic fields exhibits a similar OER increment to its magnetized multi-domain one. The evidence is given by comparing the pH-dependent increment of OER on multi- and single-domain FM catalysts with or without a magnetic field. The intrinsic activity of a single-domain catalyst is higher than that of a multi-domain counterpart. The latter can be promoted to approach the former by the magnetization effect. Reducing the FM catalyst size into the single-domain region, the spin-polarized OER performance can be achieved without a magnetic field, illustrating an external magnetic field is not a requirement to reap the benefits of magnetic catalysts.     

电场对(4, 0)Zigzag模型单壁碳纳米管的影响

The structural and electronic properties of a (4, 0) zigzag single-walled carbon nanotube (SWCNT) under parallel and transverse electric fields with strengths of 0-1.4×10~(-2) a.u. Were studied using the density functional theory (DFT) B3LYP/6-31G~* method. Results show that the properties of the SWCNT are dependent on the external electric field. The applied external electric field strongly affects the molecular dipole moments. The induced dipole moments increase linearly with increase in the electrical field intensities. This study shows that the application of parallel and transverse electric fields results in changes in the occupied and virtual molecular orbitals (Mos) but the energy gap between the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) of this SWCNT is less sensitive to the electric field strength. The electronic spatial extent (ESE) and length of the SWCNT show small changes over the entire range of the applied electric field strengths. The natural bond orbital (NBO) electric charges on the atoms of the SWCNT show that increase in the external electric field strength increases the separation of the center of the positive and negative electric charges of the carbon nanotube.     

Effect of magnetic field on properties of AuPt particles magneto-electrodeposited on carbon paper

AuPt nano particles are bi-functional catalysts for Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER) that were taken place on air electrodes in lithium air batteries. Magnetic field has been applied during electrodeposition for the preparation of AuPt particles. With the increase of the magnetic flux density under constant current density, the grain size decreases from about 1μm to 200nm and the activity of the AuPt catalyst increases. The magnetic field oriented vertical to the electric field has a promotion effect on increasing the catalytic ability of AuPt/carbon electrode. By pulse plating, the grain size decreases to about 100nm. By adjusting parameters of the electric field and the magnetic field, controllable in-situ preparation of AuPt catalyst with various morphology and catalytic activity could be achieved.